Dual piston accumulator

ABSTRACT

The invention relates to a dual piston accumulator which is provided, in particular, in a hydrostatic hybrid-drive system for vehicles to replace a high pressure hydro accumulator and a low pressure hydro accumulator, wherein, inside a single accumulator housing ( 2 ) which extends in the axial direction in a single piece over a high pressure part ( 4 ) and a low pressure part ( 6 ), an accumulator piston ( 8, 10 ) defines a high pressure-sided fluid chamber ( 16 ) and a low pressure-sided fluid chamber ( 18 ), both of which border an intermediate piece ( 12 ) separating the high pressure side ( 4 ) from the low pressure side ( 6 ) through which the common piston nod ( 14 ) extends for both accumulator pistons ( 8, 10 ). Said accumulator is characterised in that the wall width of the housing ( 2 ) corresponding to the high pressure pan ( 4 ) is greater than the opposite induced wall width which corresponds to the low pressure pan ( 6 ).

The invention relates to a dual piston accumulator, which is provided, in particular, in a hydrostatic hybrid drive system for vehicles to replace a high pressure hydraulic accumulator and a low pressure hydraulic accumulator.

In light of the scarcity of natural resources and the increasing impact of CO₂ on the environment, the current trend in automotive engineering is to use hybrid drive systems, which store the electric energy generated in braking modes and recover drive energy from the stored energy in order to provide assistance to the vehicle for the drive mode and, in particular, for accelerating processes. This strategy offers the possibility of decreasing the drive power of the internal combustion engine, which serves as the primary drive, for comparable road performance. The result of such a “downsizing” is not only a reduction in the fuel consumption, but it also raises the possibility of assigning the vehicles concerned to a lower emissions class that satisfies a lower-cost road tax category.

These goals can also be achieved not only with electric motor powered hybrids, but it is also possible to use hydrostatic hybrid systems owing to the high energy density of hydraulic systems. Such a hydrostatic drive system with recovery of the braking energy is disclosed, for example, in document DE 10 2005 060 994 A1.

The operational performance of such a hydrostatic hybrid system can be optimized by using a dual piston accumulator, instead of a high pressure hydraulic accumulator and a separate low pressure hydraulic accumulator. This approach makes it possible for the design to be more compact, as compared to a design using separate accumulators. In addition to compactness, the current trend is to reduce the structural weight as much as possible for systems that are installed into vehicles. Dual piston accumulators of the conventional design type, as described, for example, in document U.S. Pat. No. 6,202,753 B1 for use in deep water drilling operations, do not meet these requirements.

Based on the aforementioned prior art, the object of the present invention is to provide a dual piston accumulator that is distinguished by not only its compactness, but also its extremely low structural weight.

This object is achieved with a dual piston accumulator having the features specified in claim 1 in its entirety.

Therefore, an essential feature of the present invention lies in the fact that the wall thickness of the accumulator housing corresponding to the high pressure component is greater than the opposite reduced wall thickness that corresponds to the low pressure component. Not only does this feature reduce the weight of the accumulator housing, but it also uses the material of the housing in an optimal way in that the wall thickness in the high pressure component is adapted to the pressure level corresponding to the high pressure side, whereas the wall thickness in the low pressure component corresponding to the low pressure level prevailing in said low pressure component is considerably less. Since the accumulator housing still extends in one piece over the high pressure component and the low pressure component of the accumulator, it is possible to attain not only a design that is lightweight, but also a module that is as compact as possible.

It is especially advantageous for the arrangement to be configured in such a way that the accumulator housing is a one-piece component that forms an inner cylinder, which extends continuously without a shoulder from the high pressure component to the low pressure component and in which both accumulator pistons exhibiting the same piston diameter are guided. Such a component with a continuous inner cylinder without a shoulder can be produced as a deep drawn part or as a stamping part in such a way that its weight is significantly reduced.

It is especially advantageous for the arrangement to be configured in such a way that the intermediate piece is fastened on the end region of that section of the inner cylinder that exhibits the greater wall thickness. Owing to the connection of the intermediate piece to the housing section exhibiting the greater wall thickness, a structurally rigid securing of the intermediate piece is ensured.

In especially advantageous exemplary embodiments, the intermediate piece is an annular body with a radially external cylindrical surface, which rests against the inner surface of the inner cylinder to form a seal and is connected therewith at least at one attachment point.

In this respect, the arrangement can be configured in such a way that the cylindrical surface of the intermediate piece has at least one depression, preferably an annular groove, with which a notch, which is formed in the housing wall, engages. This arrangement makes it feasible to positionally secure the intermediate piece at a low production cost.

As an alternative, at least one radial borehole may be provided for positionally securing the intermediate piece in the cylindrical surface of the intermediate piece; and a mounting bolt or a mounting screw, which is inserted or screwed in from the outside of the accumulator housing, penetrates this radial borehole.

Furthermore, the arrangement can be configured in such a way that the wall of the inner cylinder and the cylindrical surface of the intermediate piece may have mutually aligned depressions for the engagement of an insert ring.

In such exemplary embodiments, the annular body of the intermediate piece can consist of two annular body parts that are screwed together. In this case, each annular body part forms a part of the cylindrical surface resting against the inner cylinder, where the depression in the cylindrical surface of the one annular body part is open in the direction of the other annular body part and can be closed by this other annular body part. In this type of design, the assembly may be performed in such a way that the inlay part is moved into position on an annular body part before the intermediate piece is completed with the second annular body part.

The invention is explained in detail below by means of exemplary embodiments that are depicted in the drawings. Referring to the drawings:

FIG. 1 is a longitudinal sectional view of an exemplary embodiment of the dual piston accumulator that is scaled down in size by about a factor of 4 compared to a practical embodiment; and in this view the piston positions correspond to the unloaded state of the high pressure side;

FIG. 2 is a view that corresponds to the one in FIG. 1, but rotated by 90° about the longitudinal axis and with piston positions that correspond to the largest volume of the fluid chamber of the high pressure side; and

FIGS. 3 to 7 are in each instance an enlarged detail of the area designated as A in FIG. 2.

FIGS. 1 and 2 show a dual piston accumulator according to an exemplary embodiment of the invention, wherein the accumulator housing is designated as 2. The accumulator housing 2 forms an inner cylinder, which is designed in one piece continuously from a high pressure side 4 without a shoulder, that is, with a constant inside diameter, as far as to a low pressure side 6. In this case, a high pressure-side accumulator piston 8 and a low pressure-side accumulator piston 10 are guided in such a way that they can be axially displaced in the inner cylinder. The high pressure side 4 and the low pressure side 6 are separated from each other in a fluid-tight manner by means of an intermediate piece 12, which is fixed in the inner cylinder. A piston rod 14, which is connected to both accumulator pistons 8, 10, extends in a fluid-tight manner through the intermediate piece 12. FIG. 1 shows the piston positions, in which a high pressure-side fluid chamber 16, which may be found between the accumulator piston 8 and the intermediate piece 12, has its smallest volume, whereas a low pressure-side fluid chamber 18, which may be found between the accumulator piston 10 and the intermediate piece 12, has its largest volume. Hence, the drawing in FIG. 1 corresponds to the completely unloaded state. In contrast, FIG. 2 shows piston positions that correspond to the loaded state. In this case, the low pressure-side fluid chamber 18 has the smallest volume; and the high pressure-side fluid chamber 16 has the largest volume; and, thus, the accumulator is pushed against the end 22 of the accumulator housing 2 that forms the gas side 20. This end 22 is closed, except for a port 24 for the working gas (preferably N₂), whereas the low pressure-side end of the accumulator housing 2 is open in the direction of the atmosphere.

It is clear from FIGS. 1 and 2 that the accumulator housing 2 is a single piece component that is made, for example, by a deep drawing process. In this case, the wall thickness is adapted to the high pressure-side pressure level over the longitudinal section of the high pressure side 4 and changes at the end of the high pressure side 4 into a reduced wall thickness that is adapted to the pressure level of the low pressure side 6 that is much lower than that of the high pressure side. The intermediate piece 12 is secured on the inner wall of the inner cylinder at the respective end region of the high pressure side 4, thus on the end of the region of the accumulator housing 2 that still has the greater wall thickness.

In the rotational position of the accumulator housing 2 that is shown in FIG. 1, an opening 26 that lies radially on the outside on the intermediate piece 12 is visible. This opening forms a fluid path to the high pressure-side fluid chamber 16 and, in addition, fulfills another purpose, which will be explained in detail below in conjunction with FIGS. 6 and 7. In addition, there is a fluid path in the intermediate piece 12; and this fluid path, which leads to the low pressure-side fluid chamber 18, is not visible in the drawings of the housing 2 shown in FIGS. 1 and 2.

A number of exemplary embodiments of the positional securing of the intermediate piece 12 are explained with reference to FIGS. 3 to 7. The intermediate piece 12 forms, on the whole, an annular body with a radially external cylindrical surface 30, which rests against the inner surface of the housing 2 and is sealed off from said inner surface of the housing by means of seals 32. It is self-evident that the accumulator pistons 8, 10 are also sealed off from the inner cylinder by means of the piston seals 34. FIG. 3 shows that the cylindrical surface 30 of the intermediate piece 12 has a depression 36 with which a notch 38, formed into the wall of the accumulator housing 2, engages in order to secure the intermediate piece 12.

In contrast, FIGS. 4 and 5 show embodiments in which the cylindrical surface 30 of the intermediate piece 12 has radial boreholes 40. In FIG. 4, a mounting screw 42 penetrates the borehole 40; and in FIG. 5 the mounting bolt 44 penetrates the borehole 40.

FIG. 6 shows an embodiment in which the wall of the inner cylinder of the accumulator housing 2 has a depression 46, which aligns with a depression 48 made in the cylindrical surface 30 of the intermediate piece 12. In this configuration, the position of the intermediate piece 12 can be secured by means of an insert ring 50. In this case, the insert ring is a ring made of an elastically flexible material with sufficient strength, such as spring steel, which is slotted, i.e., not closed, so that said ring can be slid through the opening, designated as 26 in FIG. 1, into the annular space that is formed by the aligned depressions 46 and 48.

The major distinction between the example shown in FIG. 7 and the example shown in FIG. 6 lies in the fact that the annular body of the intermediate piece 12 consists of two annular body parts 28 and 29, which are connected together by means of a threaded joint 52 and jointly define the radially external cylindrical surface 30. An insert ring 50 is used again for securing the intermediate piece 12 in the space formed by the depressions 46 and 48. However, the depression 48 in the annular body part 28 is designed in such a way that it is open in the direction of the other annular body part 29 and is closed by this annular body part 29 when screwed together with the annular body part 28.

It is self-evident that the invention is not limited to the depicted embodiments of the positional securing of the intermediate piece 12, but rather it is also possible to use other attachment techniques, such as welding, adhesive cementing, or the like. 

1. A dual piston accumulator, which is provided, in particular, in a hydrostatic hybrid drive system for vehicles to replace a high pressure hydraulic accumulator and a low pressure hydraulic accumulator, wherein inside a single accumulator housing (2), which extends in the axial direction in a single piece over a high pressure component (4) and a low pressure component (6), an accumulator piston (8, 10) defines a high pressure-side fluid chamber (16) and a low pressure-side fluid chamber (18), both of which border on an intermediate piece (12), which separates the high pressure side (4) from the low pressure side (6) and through which the common piston rod (14) extends for both accumulator pistons (8, 10), characterized in that the wall thickness of the housing (2) corresponding to the high pressure component (4) is greater than the opposite reduced wall thickness that corresponds to the low pressure component (6).
 2. The dual piston accumulator according to claim 1, characterized in that the accumulator housing (2) is a one-piece component that forms an inner cylinder, which extends continuously without a shoulder from the high pressure component (4) to the low pressure component (6) and in which both accumulator pistons (8, 10) exhibiting the same piston diameter are guided.
 3. The dual piston accumulator according to claim 1, characterized in that the intermediate piece (12) is fastened on the end region of that section of the inner cylinder that exhibits the greater wall thickness.
 4. The dual piston accumulator according to claims 1, characterized in that the intermediate piece (12) is an annular body with a radially external cylindrical surface (30), which rests against the inner surface of the inner cylinder to form a seal (32) and is connected therewith at least at one attachment point.
 5. The dual piston accumulator according to claims 1, characterized in that the cylindrical surface (30) of the intermediate piece (12) has at least one depression (36), preferably an annular groove, with which a notch (38), which is formed in the housing wall, engages.
 6. The dual piston accumulator according to claim 1, characterized in that the cylindrical surface (30) of the intermediate piece (12) has at least one radial borehole, through which a mounting bolt (44) or a mounting screw (42) extends.
 7. The dual piston accumulator according to claim 1, characterized in that both the wall of the inner cylinder and the cylindrical surface (30) of the intermediate piece (12) have depressions (46, 48), which align with each other, for the engagement of an insert ring (50).
 8. The dual piston accumulator according to claim 1, characterized in that the annular body of the intermediate piece (12) consists of two annular body parts (28, 29) that are screwed together; and that each annular body part forms a part of the cylindrical surface (30) resting against the inner cylinder; and that the depression (48) in the cylindrical surface (30) of the one annular body part (28) is open in the direction of the other annular body part (29) and can be closed by this other annular body part (29). 